PNR stands for Passenger Name Record, a jargon used by the Indian railways to uniquely identify each reserved tickets. Each reserved tickets will have 10 digit number on it , irrespective of the ticket status is confirmed, RAC (reservation against cancellation) or waitlisted (WL).
There are two types ( formats) of reserved tickets depending on how you have done the reservation. The first is you've reserved your ticket online ( over internet ) and printed the ticket yourself. This is called e-ticket. The other type of ticket is the one you've bought from one of the many reservations counters of Indian Railways.
Whichever way you reserved , a PNR is provided on top of the ticket. Note that PNR is different from the Ticket Number or Transaction ID (e-ticket) that too are printed on the ticket.
The 10 digit PNR is printed in this fashion : 123-1234567
Now let us see the utility of PNR. The most useful of all is to enquire the PNR Status. That is checking the progress of your wait list or RAC status. For the confirmed reservations, PNR is used to find the coach number and the seat number.